The price of sending mail through the U.S. Postal Service will go up Monday. The cost of mailing a first-class letter will go up from 42 cents to 44 cents. Certified mail will go from $2.70 to $2.80, and Priority Mail flat-rate envelope will increase from $4.80 to $4.95. Customers can continue to mail letters at Monday’s prices if they had purchased Forever Stamps, which are honored whenever they are used with no need for additional postage for a one-ounce letter.